1. Field of the Invention
The present invention relates to a semiconductor ceramic, in particular to a semiconductor ceramic and a semiconductor ceramic element each of which has a positive resistance temperature characteristic. This invention also relates to a circuit protection element formed by using the semiconductor ceramic element.
2. Description of the Related Art
Conventionally, a semiconductor element has been used to protect an electric circuit from being damaged by an overcurrent or has been used as a demagnetization element for a color television receiver. Such a semiconductor element is formed by providing electrodes on a semiconductor ceramic having a positive resistance temperature characteristic (hereinafter referred to as PTC characteristic) indicating that its resistance will suddenly become high as soon as its temperature arrives at the Curie temperature or higher. As a semiconductor ceramic for use in forming the semiconductor element, what has been used is a barium titanate ceramic whose specific resistance under room temperature is relatively small and whose resistance temperature coefficient is relatively large.
The barium titanate semiconductor ceramic is desired to have an improved resistance temperature characteristic. Further, it is desired to have a reduced specific resistance under the room temperature (hereinafter referred to as room temperature specific resistance) and to have an improved voltage proof characteristic. Accordingly, various researches have been conducted in order to satisfy these requirements.
For example, Japanese Unexamined Patent Application Publication No. 3-54165 has disclosed a barium titanate semiconductor ceramic which contains as its main components 45 to 87 mol % of BaTiO3, 3 to 20 mol % of PbTiO3,5 to 20 mol % of SrTiO3 and 5 to 15 mol % of CaTiO3, and also contains as additives a semiconduction agent which is one substance selected from a group consisting of Sb, Bi, Nb, Ta and some rare earth elements, 0.2 to 0.5 mol % of Mn, and 0 to 0.45 mol % of SiO2. By virtue of such a composition, the room temperature specific resistance may be controlled within a range of 3 to 10 xcexa9xc2x7cm.
Further, Japanese Unexamined Patent Application Publication No. 3-88770 has disclosed a semiconductor ceramic which contains as its main components 45 to 85 mol % of BaTiO3, 1 to 20 mol % of PbTiO3, 1 to 20 mol % of SrTiO3 and 5 to 20 mol % of CaTiO3, and also contains as additives 0.1 to 0.3 mol % of a semiconduction agent, 0.006 to 0.025 mol % of Mn and 0.1 to 1 mol % of SiO2. By using such a composition and by adjusting the particle diameter of the mixture, its room temperature specific resistance may be controlled within a range of 4 to 8 xcexa9xc2x7cm, and a gradient of a resistance temperature characteristic may be kept at 9%/xc2x0 C. or higher.
Moreover, Japanese Unexamined Patent Application Publication No. 9-22801 has taught that if the percentage of the intergranular resistance value occupying an overall resistance value consisting of the intergranular resistance value and the transgranular resistance value of the semiconductor ceramic crystal particles is controlled at 20% or lower, it is possible to improve the voltage proof characteristic of the semiconductor ceramic.
The commercial market in the field of the present invention has an increasingly high demand with respect to the semiconductor ceramic having PTC characteristic, which is to further improve its resistance temperature characteristic, to further reduce its room temperature specific resistance and to further improve its voltage proof characteristic. In particular, it has been desired that the semiconductor ceramic for use in forming circuit protection should have a lower specific resistance at the room temperature in order to reduce electric power consumption.
It is an object of the present invention to provide a semiconductor ceramic and a semiconductor ceramic element having a room temperature specific resistance of 3 xcexa9xc2x7cm or lower and having a resistance temperature characteristic of 9%/xc2x0 C. or more.
The present invention has been accomplished in view of the above object.
A semiconductor ceramic according to a first aspect of the present invention has a positive resistance temperature coefficient characterized in that the ratio R1/(R1+R2), in which R1 is the transgranular resistance value of the crystal particles and R2 is the intergranular resistance value of the crystal particles and R1+R2 is the overall resistance value representing the sum of R1 and R2 is about 0.35 to 0.85.
By forming the above ratio of the transgranular resistance value to the overall resistance value, it is possible to obtain a semiconductor ceramic element having a room temperature specific resistance of 3 xcexa9xc2x7cm or lower and having a resistance temperature characteristic of 9%/xc2x0 C. or more.
Further, a semiconductor ceramic according to a second aspect of the present invention is characterized in that the crystal particles are referred to contain barium titanate as its main component. By forming the above composition, it is possible to obtain a semiconductor ceramic having an excellent PTC characteristic.
Moreover, a semiconductor ceramic element according to a third aspect of the present invention is characterized in that it is formed by providing electrodes on a semiconductor ceramic obtained according to the first or second aspect of the present invention.
Further, a circuit protection element made according to a fourth aspect of the present invention is preferred to be formed by using a semiconductor ceramic according to the third aspect of the present invention.
By using the semiconductor ceramic in this manner, one can more effectively utilize the semiconductor ceramic element of the present invention having a low specific resistance under a room temperature.